1. Field of the Invention
The present invention relates to an image display apparatus such as an active matrix driving liquid crystal display apparatus.
2. Description of the Related Art
As an image display apparatus, a display apparatus employing an active matrix driving mechanism has been known. This image display apparatus includes a plurality of scanning signal lines and a plurality of data signal lines. Each of the scanning signal lines intersects respective data signal lines. Pixels are provided in the portions surrounded by two adjacent scanning signal lines and two adjacent data signal lines, and are arranged into a matrix pattern.
In the case of a liquid crystal display for instance, as shown in FIG. 23, each pixel Z comprises a transistor TR such as a TFT (Thin Film Transistor) or an FET as a driving element, a pixel capacitor C.sub.P, and an storage capacitor Cs which is added as necessary. As shown in this figure, a data signal line 101 is connected to one of the electrodes of the pixel capacitor Cp through the drain and source of the transistor TR. The gate of the transistor TR is connected to a scanning signal line 102, and the other electrode of the pixel capacitor Cp (common electrode) is connected to a common power source line (not shown).
The scanning signal lines 102 are connected to a scanning driver 104 which outputs a scanning signal. The data signal lines 101 are connected to a data driver 103 which samples an image signal and transfers it to the data signal lines 101 as a display data signal. The data driver 103 outputs a display data signal to the data scanning lines 101 for every scanning signal line 102 or for every pixel Z. The transistor TR is turned on when the scanning signal line 102 is activated, and then, the display data transferred on the data signal line 101 is written in the pixel capacitor Cp and the storage capacitor Cs.
A display at the pixel Z is maintained by the electric charge written in the pixel capacitors Cp and the storage capacitor Cs. An alternating current drive (inversion driving) is required to prevent the deterioration of the pixel capacitor Cp, or of the liquid crystal. When the alternating current drive is performed by a frame cycle, flickers of 30 Hz or 25 Hz are apparent though it may be different depending on the field frequencies (or frame frequencies) of the signals. Therefore, it is common to perform a inversion driving, a so-called "frame+1H line", in which the polarity of an electric charge supplied to the pixel capacitor is inverted by every horizontal scanning in addition to the frame inversion.
In the display apparatus requiring alternating current driving, such as a liquid crystal display apparatus, even if the contents (information) of adisplayed image remain unchanged, a data driver needs to supply data signals to the data signal lines periodically for writing data in each of the pixels. This means a large amount of electric current is required for display.
This type of display apparatus has been increasingly used for the displays of hand-held information terminals in recent years. Since these hand-held information terminals are used outdoors and required to be driven with batteries or the like, it is preferable to lower their power consumption.
To solve this problem, liquid crystal display apparatuses capable of a binary display (two-tone display), which do not need to output data from a data driver periodically with the alternating current driving of the common electrode, have been proposed (Japanese Laid-Open Patent publication Nos. 58-143389 and 59-155893). These apparatuses includes data storage means and control means within each pixel. The data signal output on the data signal line from the data driver is input to the data storage means and the data is stored therein. States of the control means are controlled with the data, and the control means supplies the pixel electrode with a signal as long as the display data remains unchanged. Moreover, a display apparatus capable of imperfectly performing multi-tone display in a similar mechanism has been also proposed (Japanese Laid-Open Patent Publication No. 59-65879).
However, these proposed liquid crystal display apparatuses are essentially for the binary display (two-scale display). Although Japanese Laid-Open Patent Publication No. 59-65879 suggests a possibility for the multi-scale display, the circuit configuration of each pixel will become complicated and the power consumption can not be lowered sufficiently.